Method and system utilizing a disposable transmitter for monitoring a patient{3 s condition

ABSTRACT

A method and system for monitoring a physical condition of a patient employing a radio frequency communications link between a disposable transmitter carried by the patient and a display unit. The display unit may be portable and include a receiving antenna. In other embodiments, the receiving antenna may be associated with the bed of the patient and the condition monitored by the electrical connection of a portable display unit thereto. In other embodiments, the receiving antenna associated with the bed of the patient may be wired to a remote console.

'United States Patent [191 Baessler [76] Inventor: Lee R. Baessler, 706 Anderson St.,

Manhattan Beach, Calif. 90266 [22] Filed: Aug. 23, 1973 [21] Appl. No.: 390,759

[52] U.S. Cl. 128/2 H; 73/342; 73/362 AR;

128/2.1 A; 324/181; 340/228 R [51] Int. Cl. A61B 5/00 [58] Field of Search 128/2 H, 2 R, 2 S, 2.05 R,

128/2.06 S, 2.1 A, 2.1 R; 73/340-342, 359361, 362 R, 362 CP, 362 AR, 362 SC; 324/181, 186, 78 D;340/179,182,183,188 CH, 227 R, 228 F, 228 R Nov. 25, 1975 3,477,292 11/1969 Thornton 73/362 AR 3,534,728 10/1970 Barrows 128/2.1 A 3,566,384 2/1971 Smith et al. 340/188 CH 3,572,316 3/1971 Vogelman et al. l28/2.1 A 3,638,642 2/1972 Heflin, Sr l28/2.1 A 3,735,347 5/1973 Whitney et a1... 324/78 D 3,811,428 5/1974 VanHorn et al l28/2.06 F

Primary ExaminerWilliam E, Kamm Attorney, Agent, or Firm-Burns, Doane, Swecker & Mathis [57] ABSTRACT A method and system for monitoring a physical condition of a patient employing a radio frequency communications link between a disposable transmitter carried by the patient and a display unit. The display unit may be portable and include a receiving antenna. In other embodiments, the receiving antenna may be associated with the bed of the patient and the condition monitored by the electrical connection of a portable display unit thereto. In other embodiments, the receiving antenna associated with the bed of the patient may be wired to a remote console.

9 Claims, 10 Drawing Figures Sheet 1 of 4 3,921,621

US. Patent Nov. 25, 1975 US. Patent Nov. 25, 1975 Sheet20f4 3,921,621

REF

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n ICU FIGIO METHOD AND SYSTEM UTILIZING A DISPOSABLE TRANSMITTER FOR MONITORING A PATIENTS CONDITION BACKGROUND OF THE INVENTION Body temperature has long been recognized as being of clinical significance in the diagnosis and treatment of disease; and in hospitals today, the temperature of each patient is generally recorded several times each day. The current methods of obtaining body temperature include a small mechanical thermometer or a thermister probe administered orally or rectally and connected to an electronic display. The mechanical thermometer generally requires a three to five minute stabilization period before an accurate reading can be obtained. The thermister probe is generally faster than the thermometer but generally requires a period of thirty seconds to one minute for stabilization.

Both mechanical thermometers and thermister probes are difficult to use on comatose patients and patients who require frequent measurements of their body temperature. In addition, the time spent by a nurse making temperature rounds may be quite significant and diverts the attention of the nurse from other and perhaps much more significant duties. In the event that the attention of the nurse is needed elsewhere, temperature rounds may receive a low priority and be omitted to the consternation of the diagnosising physician when later faced with the hiatus in data.

A further problem exists in the need for reuse and thus sterilization of mechanical thermometers and thermister probes. Not only is the threat of contamination ever present with reusable devices, but the expenditures of time necessary to effect and maintain a sterile condition may be of real significance.

It is accordingly an object of the present invention to obviate these and other deficiencies of generally known prior temperature systems and to provide a novel method and temperature monitoring system.

Another object of the present invention is to provide a novel method and body temperature measuring system in which the patient need not be disturbed.

It is still another object of the present invention to provide a novel method and body temperature measuring system in which the body temperature of a patient may be continuously monitored.

Still another object of the present invention is to provide a novel method and body temperature measuring system in which sequential measurements may be obtained from different patients without a prolonged stabilization or warmup period.

A further object of the present invention is to provide a novel method and body temperature measuring system in which the temperature probes are disposable thereby obviating the need for sterilization.

Still a further object of the present invention is to provide a novel method and body temperature measuring system in which the temperature probe is carried by the body of the patient so that the patient may be moved throughout the hospital without disturbing the efficacy of the system.

Still a further object of the present invention is to provide a novel disposable condition responsive transmitter.

Yet a further object of the present invention is to provide a novel portable sensing unit.

These and many other objects and advantages of the present invention will be readily apparent from the claims and from a perusal of the following detailed description when read in conjunction with the appended drawings.

THE DRAWINGS FIG. 1 is'a pictorial representation illustrating the use of one embodiment of the present invention;

FIG. 2 is a pictorial representation of one embodiment of the novel sensor unit of the system illustrated in FIG. 1;

FIG. 3 is a pictorial representation of one embodiment of the portable display utilized in the system illustrated in FIG. 1;

FIG. 4 is a schematic circuit diagram of the transmitter of FIG. 2;

FIG. 5 is a timing diagram illustrating waveforms appearing at various places in the circuit of FIG. 3;

FIG. 6 is a functional block diagram of the portable display unit of FIG. 3;

FIG. 7 is a pictorial representation illustrating one embodiment of a receiving antenna of the present invention;

FIG. 8 is a pictorial representation of a second embodiment of the receiving antenna of the present invention;

FIG. 9 is a pictorial representation of one embodiment of a portable display unit for use with the antennae of FIGS. 7 and 8 and,

FIG. 10 is a pictorial representation of a remote display .unit for use with the receiver antennae of FIGS. 7 and 8.

DETAILED DESCRIPTION The operation of one embodiment of the system of the present invention may be understood with reference to FIG. 1 wherein patients 20 and 22 are illustrated respectively in hospital beds 24 and 26. Each of the patients 20 and 22 may be provided with a condition responsive transmitter unit 28 such as illustrated in greater detail in FIG. 2. As shown in FIG. 2, each of the transmitter units 28 may include a battery pack 30 such as the Mallory type SR-49l2 and may be connected through a flexible electronic conductor 32 to the transmitter unit 34. The transmitter unit 34 may be provided with an extension such as the illustrated flexible wires 36 which terminates in a suitable conventional thermocouple 38.

Both the battery pack 30 and the transmitter unit 34 may be about 1 inch in width and length and about A inch to inch in thickness. These units may, of course, be integrally housed if desired. The small size of the units facilitates the securing thereof to the body of the patient, for example, by means of a strip of adhesive tape 40. The unit may, for example, be secured as illustrated in FIG. 1 to the rib cage of the patient with the thermal couple 38 extending upwardly into the annpit of the patient. When secured to the patient in this fashion upon the admission of the patient to the hospital, the unit illustrated in FIG. 2 may accompany the patient throughout the hospital without causing any discomfort to the patient.

With continued reference to FIG. 1, the temperature of the individual patients may be obtained in a very short time period, e.g., five seconds or less, by a nurse 42 who positions a rod antenna 44 in proximity to the transmitter unit of FIG. 2. As illustrated in greater detail in FIG. 3, the rod antenna 44 may be attached by means of a flexible electrical connection 46 to a handheld display unit 48. The display unit 48 is desirably provided with an indicator such as the pointer and scale illustrated in FIG. 3.

The portable unit 48 desirably includes an on/off switch 50 and may be provided with an indicator 52 such as a light emitting diode to indicate reception of a signal by the unit. The display unit 48 may also include a test switch 54 by which an internally generated signal may be utilized to test the deflection of the pointer on the scale for calibration purposes. In addition, a second indicator 56 such as a light emitting diode may be provided to indicate an acceptable level of battery voltage.

The operation of the system as described above may be understood with reference to FIG. 1. The patient, at the time of admission to the hospital, may be provided with one of the disposable transmitting units such as il-' lustrated in FIG. 2. The transmitting unit of FIG. 2 will stabilize within a few minutes and will thereafter continuously transmits a temperature responsive signal. The temperature of the patient may thereafter be obtained almost instantly by the placing of the receiving antenna 44 of the portable unit illustrated in FIG. 3 in proximity to the transmitting unit. The nurse may thus simply walk through a ward with the portable display unit in hand and sequentially place the rod antenna 44 in proximity to the transmitting unit 28 to obtain the body temperature of the patient. This temperature may at that time be recorded on the temperature chart 58 customarily provided at bedside and the nurse may immediately proceed to the next patient. In this manner, the time required to obtain the temperature readings is materially reduced and, more importantly, the individual patients need not be disturbed.

The battery pack of the transmitting unit 28 will provide sufficient power to operate the unit for seven to ten days and may be replaced at the end of that period should the patients committment be prolonged or, alternatively, disposed of upon the discharge of the patient.

With reference now to FIG. 4 wherein the transmitter unit of FIG. 2 is schematically illustrated, the battery pack may be connected across a first astable multivibrator 60 and through the emitter-collector path of an NPN transistor Q1 across a second astable multivibrator 62.

The first astable multivibrator 60 may include a pair of common emitter NPN transistors Q2 and Q3 having base and collector electrodes cross-connected through capacitors 64 and 66. In addition to the capacitors 64 and 66, the RC time constants controlling the on/off ratio or duty cycle of the two transistors Q2 and Q3 are controlled by a timing resistor 70 and a thermistor74 connected in a manner well known in the art.

The multivibrator 60 is continuously connected to the source 30 of direct current and is thus continually operating. With the values of the resistor 70 and the thermistor 74 equal, the output from the multivibrator 70 as taken from the collector electrode of the transistor Q3 will be as illustrated in waveform A in FIG. 5. If the value of the thermister 74 is twice that of the value of the resistor 70 due to the body temperature of the patient, the output waveform A of the multivibrator 60 will be as illustrated as waveform A in FIG. 5. If the value of the thermistor 74 is half the value of the resistor 70. the output waveform A will be as illustrated as waveform A in FIG. 5.

The output waveform A of the multivibrator is coupled through the parallel connection of a capacitor 76 and a resistor 78 to the base electrode of the transistor Q1 which serves as a switch to apply directcurrent from the source 30 to the second astable multivibrator 62. The multivibrator 62 may comprise a pair of common emitter NPN transistors Q4 and Q5 having the base and collector electrodes thereof cross-connected through capacitors 80 and 82. In addition to the conventional biasing and timing resistors 84, 86, 88 and 90, the collector electrode of the transistor Q4 may be con'-- nected to the source 30 through an inductor'92 which acts as an electromagnetic wave energy radiating element.

Inasmuch as the value of the resistors 84, 86, 88 and 90 are fixed, the frequency of operation of the multivibrator 62 is nominally a constant and the values are desirably selected to provide a 50 percent duty cycle or on/off ratio for the two transistors. The operating frequency is selected to be at least 1,000 times that of the operating frequency of the multivibrator 60 and is de-. sirably on the order of 500 KHZ.

In operation, the output signal A from the multivibrator 60 controlls the conduction of the transistor Q1 to thereby control the application of power to the multivi- I brator 62. This on/off modulation of the output signal I from the multivibrator 62 thus contains the intelligence as to the body temperature of the patient upon which the transmitting unit 28 of FIG. 1 is located.

The transistors Q1 Q5 are desirably all physically located on a single wafer such as the RCA device CA 3086. The capacitors and timing resistors are desir-.

ably located in close physical proximity and may be encapsulated in epoxy to eliminate the thermal gradient.

Thus ambient temperature and battery voltage variaa tions will not affect the quality of the duty cycle intelli-. gence transmitted for the reason that variations in fre-, quency will not affect the duty. cycle.

With reference now to FIG. 6 where the display unit 48 of FIGS. 1 and 3 is illustrated in functional block di- F counter 114. The output signal from the counter 114 is applied to one input terminal 116 of a binary latch element 118 and the output signal from the binary latch element 118 may be applied through a digital-to-analog converter 120 to a suitable conventional display 122. The phase locked loop 106 provides an output signal by way of a terminal 124 to a counter 126. and receives impulses therefrom by way of a terminal 128. The phase locked loop 106 also provides an output signal to the control pulse generator 110 by way of an input terminal and to the bidirectional counter 114 by way of an input terminal 132.

In operation and with continued reference to FIG. 6, the electromagnetic wave energy signal received by the antenna 44 is amplified in the amplifier 100 and applied to the detector 102 wherein the envelope is detected and applied to the phase locked loop 106. The phase locked loop 106 may be any suitable conventional type such as the RCA type CD-4046A utilizing a pulse edge controlled phase comparator. The output signal from the phase locked loop 106 applied to the counter 114 is thus a predetermined multiple, e.g., 1,024, of the frequency of the signal on the input terminal 104 by virtue of the connection of the phase locked loop 106 to the counter 126.

The output signal from the detector 102 is applied to the control input terminal 112 of the bidirectional counter 114 so that the pulses applied from the phase locked loop 106 to the input terminal 132 are counted upwardly during the off or reference signal level and downwardly in the presence of the on or condition responsive level signal. As a result, the state of the counter 114 at the end of each cycle of the temperature data reflects the difference between the reference and condition responsive portions of the signal, i.e., the body temperature of the patient.

At the end of each cycle, the control pulse generator effects the transfer of the count in the counter 114 to the binary latch element by the application of a pulse to the input terminal 136. Immediately thereafter, the control pulse generator 110 effects the resetting of the bidirectional counter 114 by the application of a pulse to an input terminal 134. The number stored in the bi nary latch element may be digitally displayed by any suitable conventional means or may, as illustrated, be converted to analog form in the digital-to-analog converter 120 and displayed in the form of pointer deflection in a display 122 such as that illustrated in FIG. 3.

In summary, the phase locked loop 106 is utilized to provide a predetermined phased number of output pulses during each complete cycle of the received data irrespective of variations in the frequency at which the data is received. The detected modulation controls the bidirectional counter so that it is incremented in one direction for a reference period and in the opposite direction during the condition responsive period. The count thus left in the counter 114 at the end of each cycle thus reflects the duty cycle of the multivibrator 62 of FIG. 4 and thus the body temperature of the patient. At the end of each cycle, the count in the counter 114 is transferred to the binary latch element 118 for subsequent display and almost immediately thereafter the counter is reset for the next cycle of the temperature data.

For practical considerations, the thermistor 74 of FIG. 4 was selected with a 25C resistance of 100 K ohms and a temperature coefficient of 4.9 percent per degree C. The reference resistor 70 of the circuit of FIG. 4 was selected so that the value of the thermistor will be 80 percent thereof at a temperature of 101F. To provide a temperature range of 96 to 106F, the resistance of the thermistor 74 will vary from about 90 percent to about 70 percent respectively of the value of the reference resistor 70. Since the total cycle of the multivibrator 60 of FIG. 4 is divided into 1,024 equal parts by means of the phase locked loop 106 and counter 126, the portion of the cycle representing the temperature range from 96 to 106F will be divided into approximately 100 equal portions. Thus the temperature indicated by the binary number remaining in the counter 114 will reflect the sensed body tempera ture over a fairly wide variation in the frequency of the multivibrator 60 and critical component selection is thus obviated.

The system as described in connection with FIGS. l-3 may be modified to transfer the receiving antenna 44 of FIGS. 1 and 3 from the display unit 48 to the individual patient. For example, and with reference to FIG. 7, the receiving antenna may comprise a loop antenna disposed between the upper surface of the mattress of a patients bed and the mattress cover. The size of the antenna wire 140 may be sufficiently small in diameter so that the presence of the receiving antenna will cause no discomfort to the patient. The antenna 140 may be of such flexible material that it can conform to the configuration of the bed as the hospital bed is adjusted for the patients comfort. The loop antenna may terminate in any suitable conventional quick-connect plug or jack 142 which may be permanently mounted on a portion of the hospital bed or which may be removably secured thereto by any suitable conventional fastening means such as a spring clip or the like.

In another embodiment, the receiving antenna may be, as illustrated in FIG. 8, carried by the headboard 144 of the patients bed. In this embodiment, the antenna 146 may be adhesively secured to the head-board and may terminate in any suitable conventional quickdisconnect plug or jack 148. The antenna 146 may be made integrally with the bed structure and the plug 148 permanently attached thereto. Alternatively, the plug 148 may be removably secured to the bed by any suitable conventional means such as a spring clip.

With the installation of an antenna in association with each of the patients beds, the display unit of FIG. 3 may be modified as shown in FIG. 9 wherein the rod antenna 44 of FIG. 3 has been replaced by an appropriately configured connector 150 adapted for mating with the plugs 142 and .148 described in connection with FIGS. 7 and 8. In this manner, the nurse or hospital attendant need not approach the patient but may simply establish the electical connection to instantly read the body temperature thereof.

As shown in FIG. 9, the display or readout of the display unit 48 may include a digital readout in lieu of the pointer and scale or meter movement illustrated in FIG. 3. In addition, the display unit 48 may be provided with a telescoping rigid antenna so that the display unit and receiving antenna may be operated with one hand. If desired, the electrical conductor 46 may be connected to the display unit 48 by means of a suitable conventional connector 152 so that the display unit may be adapted both for connection to permanently installed antennae or to a hand-held antenna of the type illustrated in FIG. 3.

In a further embodiment, the receiving antennae illustrated in FIGS. 7 and 8 may be hard wired to a remote display unit 154 such as illustrated in FIG. 10. With reference to FIG. 10, the display unit 154 may be combined with other monitoring apparatus in intensive care units or may exist separately therefrom. The display unit 154 is desirably provided with an indicator for each of the receiving antennae RA RA RA,,. Alternatively, however, a single digital display may be provided with the receiving antennae sequentially connected thereto by an appropriate electronic switching circuit. In a display of this type, it is necessary that an indicator also be provided as to the receiving antenna from which the signal is received.

ADVANTAGES AND SCOPE OF INVENTION It is apparent from the foregoing that the relatively low powered disposable transmitting unit of the present invention may be adapted to a number of different systems. The disposable character of the transmitting unit permits a unit to be secured to each patient upon admission to the hospital and to remain with the patient during his entire confinement. In this manner, the patient may leave the bed without disturbing the system and may be transferred from bed to bed Without disruption of the system.

The transmitting unit carred by the patient may be utilized to convey data as to the patients body temperature or some other condition to a portable display unit carried by a nurse or hospital attendant from bed to bed. The patients temperature may thus be monitored without disturbing the patient and the speed with which temperature rounds may be made is significantly increased due to the fact that the transmitting unit is continuously operable.

In other embodiments, the receiving antenna may be associated with the bed of the patient and the portable unit selectively connected to the receiving antenna. In still other embodiments, the receiving antennae, associated with each bed of the patient may be hard wired to a remote console for monitoring thus eliminating the need for temperature rounds. By the use of the present invention, the accuracy of the temperature measurements is materially increased and the necessity for equipment sterilization is eliminated.

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

l. A system for monitoring the body temperature of a plurality of patients comprising:

I a plurality of solid state electromagnetic wave energy transmitters each adapted to be carried by the body of a patient whose body temperature is to be monitored;

means for modulating the duty cycle of the electromagnetic wave energy transmitted to each of the solid state transmitters as a function of the body temperature of the patient by which the transmitter is carried;

a plurality of antennae each located in physical proximity to one of the patients to receive the modulated electromagnetic wave energy transmitted from the one of said plurality of transmitters carried by that patient, each of said antennae being connected to a plug; and, portable monitor having means selectively connected to the plug of one of said plurality of receiving antennae to monitor the duty cycle of the received electromagnetic wave energy to thereby 65 monitor the body temperature of a selected one of the patient, said monitor means including: means for detecting the duration of a cycle;

means for detecting the duty cycle ofrthe'detected cycle;

phase locked loop means for providing a digital sig- I nal variable in pulse repetition rate in response to said cycle duration detecting means to provide a predetermined number of pulses within the duration of I the detected cycle; and,

means responsive to said duty cycle means and to said digital signal for manifesting the value of the a sensed condition, said means including:

a bi-directional counter for counting in one direc-. tion responsively to the pulses occurring in said digital signal during the duty portion of a cycle of the received signal and for counting in the other direction during the remainder of the same cycle; i

storage means; and,

a control pulse generator foreffecting the transfer.

of the count in said bi-directional counterto said storage means and for effecting the resetting of said bi-directional counter at the end of each cycle. I i 2. The system of claim 1 wherein each of said plurality of receiving antennae comprises a flexible loop anr tenna disposable between the mattress of the patients bed and the bedding therefor without discomfort to the patient.

3. The system of claim 1 wherein each of saidplurality of transmitters includes a source of direct current a adapted to be secured to the body of a patient;

a solid state electromagnetic wave energy transmitting device adapted to be independently secured to the body of a patient and flexibly connected to said direct current source; and, a temperature responsive element connected to said transmitting device to modulate duty cycle of the the transmitted electromagnetic wave energy'as a 1 function of the temperature of the patient.

4. The system of claim 3 wherein said transmitter comprises: g

a first astable multivibrator responsive in duty cycle to the condition sensed; and, p

a second astable multivibrator having a frequency at least one thousand times greater than the frequency of said first astable multivibrator, said second astable multivibrator being enabled responastable multivibrator to the sensed condition due to] ambient temperature is significantly reduced.

6. A system for monitoring the body temperatureof I a plurality of patients comprising:

a plurality of solid state electromagnetic wave energy transmitters each adapted to be carried by the body of a patient whose body temperature is to be monitored;

means for modulating the duty cycle of the electro-- cycle duration and duty cycle, the antenna of said portable receiver being selectively positioned in proximity to a selected one of said plurality of transmitters to monitor the body temperature of a selected one of the patient, said monitor including:

a solid state electromagnetic wave energy transmitting device adapted to be independently secured to the body of a patient and flexibly connected to said direct current source; and,

means for detecting the duration of a cycle of the received signal;

means for detecting the duty cycle of the detected cycle;

means for providing a digital signal variable in pulse repetition rate in response to said cycle duration of the detected cycle; and

means responsive to said duty cycle detecting means and to said digital signal for manifesting the value of the sensed condition.

7. The system of claim 6 wherein the antenna of said portable monitor is a rod antenna connected to said portable monitor by a flexible electrical connector.

8. The system of claim 6 wherein each of said plurality of transmitters includes a source of direct current adapted to be secured to the body of a patient; 0

a temperature responsive element connected to said transmitting device to modulate the transmitted electromagnetic wave energy as a function of the temperature of the patient.

9. The system of claim 8 wherein each of transmitting devices includes:

a first astable multivibrator responsive in duty cycle to said temperature responsive element; and,

a second astable multivibrator having a frequency at least one thousand times greater than frequency of said first astable multivibrator and operatively connected thereto, said second astable multivibrator being enabled responsively to said first astable multivibrator. 

1. A system for monitoring the body temperature of a plurality of patients comprising: a plurality of solid state electromagnetic wave energy transmitters each adapted to be carried by the body of a patient whose body temperature is to be monitored; means for modulating the duty cycle of the electromagnetic wave energy transmitted to each of the solid state transmitters as a function of the body temperature of the patient by which the transmitter is carried; a plurality of antennae each located in physical proximity to one of the patients to receive the modulated electromagnetic wave energy transmitted from the one of said plurality of transmitters carried by that patient, each of said antennae being connected to a plug; and, a portable monitor having means selectively connected to the plug of one of said plurality of receiving antennae to monitor the duty cycle of the received electromagnetic wave energy to thereby monitor the body temperature of a selected one of the patient, said monitor means including: means for detecting the duration of a cycle; means for detecting the duty cycle of the detected cycle; phase locked loop means for providing a digital signal variable in pulse repetition rate in response to said cycle duration detecting means to provide a predetermined number of pulses within the duration of the detected cycle; and, means responsive to said duty cycle means and to said digital signal for manifesting the value of the sensed condition, said means including: a bi-directional counter for counting in one direction responsively to the pulses occurring in said digital signal during the duty portion of a cycle of the received signal and for counting in the other direction during the remainder of the same cycle; storage means; and, a control pulse generator for effecting the transfer of the count in said bi-directional counter to said storage means and for effecting the resetting of said bi-directional counter at the end of each cycle.
 2. The system of claim 1 wherein each of said plurality of receiving antennae comprises a flexible loop antenna disposable between the mattress of the patient''s bed and the bedding therefor without discomfort to the patient.
 3. The system of claim 1 wherein each of said plurality of transmitters includes a source of direct current adapted to be secured to the body of a patient; a solid state electromagnetic wave energy transmitting device adapted to be independently secured to the body of a patient and flexibly connected to said direct current source; and, a temperature responsive element connected to said transmitting device to modulate duty cycle of the the transmitted electromagnetic wave energy as a function of the temperature of the patient.
 4. The system of claim 3 wherein said transmitter comprises: a first astable multivibrator responsive in duty cycle to the condition sensed; and, a second astable multivibrator having a frequency at least one thousand times greater than the frequency of said first astable multivibrator, said second astable multivibrator being enabled responsively to said first astable multivibrator.
 5. The transmitter of claim 4 wherein the timing elements of said first astable multivibrator are disposed in heat transmitting relationship to each other whereby distortion in the response of the duty cycle of said first astable multivibrator to the sensed condition due to ambient temperature is significantly reduced.
 6. A system for monitoring the body temperature of a plurality of patients comprising: a plurality of solid state electromagnetic wave energy transmitters each adapted to be carried by the body of a patient whose body temperature is to be monitored; means for modulating the duty cycle of the electromagnetic wave energy transmitted by each of said solid state transmitters as a function of the body temperature of the patient by which the transmItter is carried; and, a portable monitor including a receiving antenna for receiving a signal modulated by a signal variable in cycle duration and duty cycle, the antenna of said portable receiver being selectively positioned in proximity to a selected one of said plurality of transmitters to monitor the body temperature of a selected one of the patient, said monitor including: means for detecting the duration of a cycle of the received signal; means for detecting the duty cycle of the detected cycle; means for providing a digital signal variable in pulse repetition rate in response to said cycle duration of the detected cycle; and means responsive to said duty cycle detecting means and to said digital signal for manifesting the value of the sensed condition.
 7. The system of claim 6 wherein the antenna of said portable monitor is a rod antenna connected to said portable monitor by a flexible electrical connector.
 8. The system of claim 6 wherein each of said plurality of transmitters includes a source of direct current adapted to be secured to the body of a patient; a solid state electromagnetic wave energy transmitting device adapted to be independently secured to the body of a patient and flexibly connected to said direct current source; and, a temperature responsive element connected to said transmitting device to modulate the transmitted electromagnetic wave energy as a function of the temperature of the patient.
 9. The system of claim 8 wherein each of transmitting devices includes: a first astable multivibrator responsive in duty cycle to said temperature responsive element; and, a second astable multivibrator having a frequency at least one thousand times greater than frequency of said first astable multivibrator and operatively connected thereto, said second astable multivibrator being enabled responsively to said first astable multivibrator. 